CO2 removal from flue gas has been done on limited scale for long time. Currently, there is an increased emphasis on CO2 capture to reduce green house emissions.
The use of an amine wash has been the most common technology for CO2 removal. It is energy intensive due to the large amount of heat required for the regeneration of amine solution. The CO2 product is always at low pressure of around 5 to 10 psig. There are also issues about the degradation of amine solution due to the presence of O2, NOx and SO2 in the flue gas.
CO2 removal by the use of PSA technology with various solid adsorbents, such as carbon and zeolites has also been studied. Refer to the paper “Flue-Gas Carbon Capture on Carbonaceous Sorbents: Toward a Low-Cost Multifunctional Carbon Filter for Green Energy Producers” by Maciej Radoz, Xudong Hu, Kaspars Krutkramelis, and Young Shen” Ind. Eng. Chem. Res. 2008, 47, 3783-3794. This paper describes a PSA cycle for CO2 recovery using activated carbon. It teaches that carbon is superior to zeolite for this application. The regeneration of the PSA is done with vacuum or with direct-steam or hot CO2.
Vacuum regeneration increases the power required to compress the CO2 to a typical pressure of about 2000 psig. The thermal regeneration with steam or hot CO2 occurs at ambient pressure. However the compression power for CO2 remains high.
Carbon as a media for adsorption has several disadvantages. Its capacity for CO2 is low compared to the zeolites. It also has the disadvantage that it co-adsorbs N2. CO2/N2 selectivity is under 10. CO2 product is thus contaminated with significant amount of NO2, on the order of 10% or more.